An oil control ring is attached in a ring groove formed on a cylindrical outer surface of a piston to control lubricating oil on an inner surface of a cylinder in an internal combustion engine. A typical example of currently practical oil control rings is a combined oil control ring that comprises a spacer expander and a single or a pair of side rails attached to the spacer expander. In operation of the internal combustion engine, the side rail is retained in a ring groove of the piston, while the side rail is reciprocated in a cylinder along with the piston to cause an outer surface of the side rails to slide on the inner surface of the cylinder. At the same time, the spacer expander produces elastic force to press a side surface of the side rail on a radial surface of the ring groove and also outer surface of the side rail on the inner surface of the cylinder. As a result, the outer surfaces of the side rails serve to scrape an excessive amount of lubricating oil from inner surface of cylinder toward a crank chamber to form a lubricant film of appropriate thickness on the inner surface of cylinder and also to prevent lubricating oil from creeping or crawling up into a combustion chamber.
Usually, the oil control ring is attached to a piston in parallel to and beneath a top or second piston ring attached the upper portion of piston, and a spacer expander of the oil control ring is generally designated to produce a higher pressing force on inner wall of cylinder than that of the top or second ring, providing the oil control ring with stronger tension. Since it has been required that friction force between piston rings and inner wall of cylinder is decreased in order to reduce fuel consumption, the tension of spacer expander has recently been designated to be smaller. Simultaneously, oil control rings are advisable to have a close followability or matching distortion to a deformed inner surface of cylinder during operation. Accordingly, smaller tension of the spacer expander would be desirable for nimble reciprocal movement of the piston but on the other hand such a spacer expander may result in increase of lubricant consumption because it deteriorates followability in operation of the side rail to cylinder, and therefore, the outer surface of the side rail does not keep close sliding contact to the deformed inner surface of cylinder. Accordingly, there is a tendency of demanding thinner ring width in recent oil control rings that can keep good followability of side rails to inner surface of cylinder under lowered tension of spacer expander.
For example, Japanese Patent Disclosure No. 6-272763 discloses an efficient manufacture of flat wires of high alloy steel without cracks as ring materials of side rails for oil control rings. In manufacture, steel wires are forced into a circular cross section by cold drawing or warm drawing, and then formed into flat wires of thickness less than 1 mm through cold rolling or warm rolling so that the resultant flat wire has the flat side surfaces and barrel-shaped surfaces formed between the flat side surfaces. The flat wires are further formed into a coiled shape with great circular accuracy and cut into ring materials with the barrel-shaped outer and inner surfaces effective to form oil film of appropriate thickness on inner surface of cylinder.
Also, Japanese Patent Disclosure No. 2000-320672 demonstrates a combined oil control ring of thin three-piece type that produces a lower tension with good follow-up performance to unevenly deformed cylinder surface to improve airtight property. The combined oil control ring comprises a spacer expander and two side rails attached to the spacer expander in axially spaced relation to each other wherein axial distance between outer tips of the upper and lower side rails is shorter than axial distance between imaginary central planes passing through the upper and lower side rails to improve mating distortion or followability of the side rails to deformed inner surface of cylinder in operation, but it does not produce any practical effect to better the followability.
During upward stroke of piston in operation, the outer side surface of the side rail produces a frictional force on inner surface of cylinder in an adverse direction to the moving direction of piston so that the oil control ring is moved in the ring groove toward a crank chamber. At this moment, the lower side rail is in contact to and in parallel relation to a bottom surface of the ring groove, and the lower side rail is in contact to inner surface of cylinder at a substantially central portion of the barrel-shaped outer surface, while frictional force slants the upper side rail toward the crank chamber due to clearance between the oil control ring and ring groove, and deformation of the spacer expander. Accordingly, the upper side rail is brought into contact to the inner surface of cylinder at an upper point above a central portion in thickness of the outer surface.
During downward stroke of piston, the oil control ring is moved in the ring groove toward a combustion chamber, and as a result, the upper side rail is in contact to and in parallel relation to a top surface of the ring groove, and the upper side rail is in contact to the inner surface of cylinder at a substantially central portion of the barrel-shaped outer surface, while the lower side rail is brought into contact to the inner surface of cylinder at a lower point below a central portion in thickness of the outer surface.
Although the side rail comes into contact to the inner surface of cylinder under elastic force of spacer expander, it is believed that the round outer surface of the side rail can neither always produce oil film of suitable thickness on inner surface of cylinder in the inclined and non-inclined conditions of the side rail nor scrape up and down sufficient amount of lubricating oil from inner surface of cylinder. In particular, when the oil control ring is incorporated with side rails of lower tension for reducing pressing force of side rails on inner surface of cylinder, it has been found that the oil control ring would increase oil consumption.
An object of the present invention is to provide a side rail capable of performing a good oil control function under lower tension set for an oil control ring. Another object of the present invention is to provide an oil control ring assembled with such side rails under lower tension. Still another object of the present invention is to provide a side rail and combined oil control ring capable of reducing consumption of lubricating oil and fuel in internal combustion engine.